Dynamics of protein and polar lipid recruitment during lipid droplet assembly in Chlamydomonas reinhardtii

Abstract

In plants, neutral lipids are frequently synthesized and stored in seed tissues, where the assembly of lipid droplets (LDs) coincides with the accumulation of triacylglycerols (TAGs). In addition, photosynthetic, vegetative cells can form cytosolic LDs and much less information is known about the makeup and biogenesis of these LDs. Here we focus on Chlamydomonas reinhardtii as a reference model for LDs in a photosynthetic cell, because in this unicellular green alga LD dynamics can be readily manipulated by nitrogen availability. Nitrogen deprivation leads to cellular quiescence during which cell divisions cease and TAGs accumulate. The major lipid droplet protein (MLDP) forms a proteinaceous coat surrounding mature LDs. Reducing the amount of MLDP affects LD size and number, TAG breakdown and timely progression out of cellular quiescence following nitrogen resupply. Depending on nitrogen availability, MLDP recruits different proteins to LDs, tubulins in particular. Conversely, depolymerization of microtubules drastically alters the association of MLDP with LDs. LDs also contain select chloroplast envelope membrane proteins hinting at an origin of LDs, at least in part, from chloroplast membranes. Moreover, LD surface lipids are rich in de novo synthesized fatty acids, and are mainly composed of galactolipids which are typical components of chloroplast membranes. The composition of the LD membrane is altered in the absence of MLDP. Collectively, our results suggest a mechanism for LD formation in C.reinhardtii involving chloroplast envelope membranes by which specific proteins are recruited to LDs and a specialized polar lipid monolayer surrounding the LD is formed. Significance Statement Lipid droplets (LDs) are dynamic organelles of virtually every cell type and are involved in numerous metabolic and physiological processes. Many aspects of LD biology, particularly in photosynthetic cells remain obscure. Using Chlamydomonas reinhardtii as a model, we uncovered an interaction of microtubules with the major lipid droplet protein affecting protein targeting to LDs. We also provide evidence for a specialized polar lipid composition of LDs suggesting an origin of LDs from chloroplast envelope membranes.